The present invention relates to ophthalmic lenses for use in eye wear.
Multifocal lenses have been used in situations requiring spectacle correction for presbyopia. Often patients using such correction devices encounter disturbing problems inherent in many designs. These problems include image jump, limited range of clear vision, annoying reflections from the edge of lens segments and cosmetic objections. For example, conventional flat top, curve top and ribbon shaped segments have ledges which either protrude from the plastic lenses or are fused within the glass lenses. These ledges often reflect light into the wearer's eye and are cosmetically unappealing because of their obviousness to the observer.
Attempts have been made to correct these problems through the use of round segments. Such segments, however, still provide a clearly noticeable, cosmetically unattractive boundary between the near and distance areas. Many round and flat top segments provide clear vision for distance and near viewing but fail to overcome the problem of image jump when passing from the near viewing zone to the distance vision zone and vice versa. Several early attempts to deal with these problems are shown in U.S. Pat. Nos. 1,448,052 and 1,518,405.
In recent years, progressive lenses have been used to correct presbyopia. Progressive lenses besides their obvious cosmetic appeal provide a continuous range of focal powers. This benefit is partially offset by the peripheral astigmatism and distortion abberations that are unavoidably present in almost all progressive lenses. The design of modern progressive lenses often centers on reducing these unwanted distortions. U.S. Pat. Nos. 3,687,528, 3,711,191, 4,461,550, 4,484,804, 4,514,061 and 4,592,630 illustrate some of the progressive lenses known in the art.
One of the deficiencies of currently available progressive lenses is that they do not provide sphericity for near viewing. Instead they use large numbers of radii in an attempt to give a gradual progression in vertically increasing power. They make the assumption that it is necessary to provide clear vision for all near working distances. The problem which arises from this is that each progressively shorter radius leaves an area of uncorrectable distortion as it departs from the connecting point which is the vertical center line of the segment. Vision with standard progressive lenses is conceptually clear at the center line of the lens. In practice, however, the human eye typically requires an area at least 18 to 35 mm wide for horizontal field comfort.
Some progressives also include blended areas which extend all the way to the edges of the lens. These blended areas are intended to minimize distortion and achieve cosmetic appeal. The erroneous assumption made by the designers of such lenses is that patients can tolerate distance distortion. Still another deficiency of many progressive lenses is that they have narrow corridors of power increase which are difficult for opticians to correctly position, especially for reading. It is also difficult for the user to aim the lens toward the reading area.
Accordingly, it is an object of the present invention to provide an ophthalmic lens which provides clear viewing substantially without distortion in both near and distance viewing areas.
It is a further object of the present invention to provide a lens as above having an extended near viewing range when required.
It is a still further object of the present invention to provide a lens as above in which there is substantially no image jump.
It is yet another object of the present invention to provide a lens as above which is cosmetically appealing and has no visible transition lines.
It is yet another object of the present invention to provide a lens as above which can be easily fitted to the user.
These and other objects and advantages will become clearer from the following description and drawings.